Goodwill and intangibles - Composition of Finite-Lived Intangibles (Details) - USD ($) $ in Millions	Jul. 29, 2017	Jan. 28, 2017	Jul. 30, 2016
Definite-lived intangible assets:
Gross carrying amount	$ 49.8	$ 49.0	$ 49.2
Accumulated amortization	(43.6)	(36.8)	(30.0)
Net carrying amount	6.2	12.2	19.2
Intangible assets, gross	457.5	453.8	454.7
Total intangible assets, net	413.9	417.0	424.7
Definite-lived intangible liabilities:
Gross carrying amount	(114.5)	(113.9)	(114.0)
Accumulated amortization	80.5	71.7	62.0
Total	(34.0)	(42.2)	(52.0)
Unfavorable leases
Definite-lived intangible liabilities:
Gross carrying amount	(48.9)	(48.3)	(48.4)
Accumulated amortization	44.3	38.2	31.2
Total	(4.6)	(10.1)	(17.2)
Unfavorable contracts
Definite-lived intangible liabilities:
Gross carrying amount	(65.6)	(65.6)	(65.6)
Accumulated amortization	36.2	33.5	30.8
Total	(29.4)	(32.1)	(34.8)
Trade names
Definite-lived intangible assets:
Gross carrying amount	1.5	1.4	1.4
Accumulated amortization	(0.9)	(0.8)	(0.6)
Net carrying amount	0.6	0.6	0.8
Favorable leases
Definite-lived intangible assets:
Gross carrying amount	48.3	47.6	47.8
Accumulated amortization	(42.7)	(36.0)	(29.4)
Net carrying amount	5.6	11.6	18.4
Trade names
Definite-lived intangible assets:
Indefinite-lived trade names	$ 407.7	$ 404.8	$ 405.5